1. Field of the Invention
The present invention relates to a user equipment for mobile telecommunications and to a method for accessing a mobile telecommunications network.
2. Present State of the Art
According to the terminology currently adopted for the UMTS (Universal Mobile Telecommunication System) and LTE (Long Term Evolution) technologies, the term “user equipment” (or its acronym “UE”) is used to indicate any device which can be directly used by a final user to communicate by connecting to the UMTS access network. It may be a mobile phone, a smartphone, a laptop with a broadband data adapter, or any other device connecting to the Radio Base Station, as set out in the ETSI 125/136-series and 3GPP 25/36-series specifications. In general, the user equipment corresponds to the mobile station (MS) of the GSM system. For simplicity, in the present description the term “user equipment” will refer to any device which can be directly used by a final user to communicate by connecting to the radio mobile access network, regardless of the standard adopted by the mobile telecommunications network itself (GSM, UMTS, CDMA2000, W-CDMA, etc.), in that the present invention is applicable without distinction to any of such devices.
When natural calamities (earthquakes, seaquakes, floods) or exceptional events (large gatherings, important sports events, New Year's celebrations, etc.) occur, the transmission resources of mobile telecommunications networks may become insufficient to meet the increased service demand. In such overload conditions, many service requests cannot be fulfilled because the required resources are insufficient (links between network elements, service server, routing units, etc.), and the users will react to this by reiterating their requests. The access request traffic will then increase out of all proportion, resulting in the risk that the signalling network collecting such traffic might become congested. In this case, the telecommunications system will enter a critical out-of-service condition.
The most effective way of preventing the signalling network from entering such a condition, or of bringing it out if it has unfortunately entered it, consists of applying access restrictions at the very origin, i.e. in the user equipments, so that the total requests collected in the signalling network will be smaller than the quantity that would otherwise cause it to become congested.
Restriction mechanisms of such a kind have been known for a long time. For example, the GSM 02.11 specification, version 3.5.0 of January 1991, of the GSM system (Global System for Mobile communications) already defined the possibility, which still exists, of inhibiting groups of users from issuing requests for accessing the mobile telecommunications network. For this purpose, mobile telecommunications network operators divide their customers, i.e., the SIM's (Subscriber Identity Modules) associated with their networks, into different “access classes” and, if needed, they can block any access requests from the SIM's belonging to one or more classes, in rotation, through signallings which are broadcast in each cell of the network, so as to allow the SIM's belonging to other classes to find the resources necessary to gain access to the services. Users having a normal priority level are distributed among ten access classes, while users having special priority levels (generally higher) are entered into five additional classes. Emergency calls are subject to a specific priority treatment.
The same technique is used in UMTS (Universal Mobile Telecommunications System), in accordance with the 3GPP TS 25.331 specification.
As for LTE (Long-Term Evolution), the access restriction mode was refined by substantially following patent EP 2205026 to NTT DoCoMo, Inc. (priority date: 24th October 2007), as partly anticipated by patent EP 1773007, also to NTT DoCoMo, Inc. (priority date: 7th October 2005). The 3GPP TS 36.331 V8.1.0 specification of March 2008, in fact, introduced a form of restriction dependent on two parameters:                a barring probability, according to which the user equipments draw for the possibility of attempting to access the network, as will be illustrated below;        an average wait time, indicating, as will be described below, the time that the user equipments must wait, after an unsuccessful barring draw, before attempting a new draw.        
In March 2011, this restriction mode was further refined into the form set out in the 3GPP TS 36.331 V11.0.0 (2012-06) specification. The main rules will be synthetically described below without taking into account, for simplicity, the various particular cases and sub-cases tackled by the specification.
Considering that every user equipment has relevant system information (which is broadcast in every cell of the network), when the user equipment is requested to establish a connection to the network, said user equipment first verifies if in the cell it belongs to there is a barring that concerns it. If there is no barring, the equipment attempts to gain access by sending a first message to the network. If there is a barring, the user equipment performs the following phases:                a. it generates a “random probability value”;        b. if the value is below the barring probability specified by the network, it issues the request for accessing the mobile telecommunications network;        c. otherwise, it emits no signal and generates a “random_delay_factor” evenly distributed between zero and one;        d. before allowing a new attempt to access the network, a wait time must elapse, which is calculated by using the following formula (1):(0.7+0.6×random_delay_factor)×average wait time  (1)where the average wait time can be set by the network in the range of 4 seconds to 512 seconds (approx. 8.5 minutes), so that the wait time can range between a minimum value of 2.8 seconds and a maximum value of approx. 11 minutes.        
This procedure allows to prevent the network from entering a congested state, or to bring it out if it has fallen into it, thus minimizing the resource utilization in the “uplink” transmission direction (i.e., from the user equipment to the radio base station), which in such circumstances is the critical one.
Restrictions can be set distinctly for the various services offered by the network (emergency calls, data services, signalling, GSM or UMTS circuit voice services, voice and video multimedia services specifically dealt with by document 3GPP TS 24.173, etc.), with the possibility of favouring those which are considered to be more useful as the case may be. However, the procedure of the 3GPP TS 25.331 specification has a few drawbacks.
When access restrictions are established, it is very likely that the traffic is already much more intense than that for which the network has been sized; therefore, it is not advisable that the restricted mode starts by giving the possibility of immediately generating an access request to a certain percentage of users (those who are lucky at the first draw), who might make concurrent requests.
After the initial transient, when it can be assumed that the expirations of the wait times are evenly distributed over time, to draw for the possibility of attempting an access after a wait time has elapsed means to reduce the average number of access attempts per time unit. From the network's viewpoint, the same result can be attained if, instead of a draw followed by a wait time having a given duration, the system simply imposes a wait time having the same duration as the preceding alternative divided by the probability of a successful draw. From the single users' viewpoint, on the other hand, the combination of draw and wait time may be more disadvantageous for some users than for others.
Let us assume, for example, that the barring probability imposed by the network is 85%. In such a case, the probability that a user equipment will have the possibility of attempting an access at the first draw will be 15%. This is not very different from the probability of 17% that another user equipment will make eleven consecutive unsuccessful draws (0.8511≈0.17). Furthermore, at the end of a successful connection or after an unsuccessful access attempt, the user equipment that was granted access to the network can immediately draw for the possibility of requesting another access, just like any other user equipment to which the draw did not grant access for one or more consecutive times, and which therefore had to wait for one or more wait times to elapse (calculated with formula 1) prior to reattempting the draw.
Given the facts that after an unsuccessful draw it is necessary to wait for a wait time which may be as long as a few minutes, that the barring concerns the possibility of attempting an access, and that, if the possibility of making an attempt is obtained, it is not certain that the attempt will be successful (when exceptional events occur, it is very likely that the transmission resources stay close to saturation, if not congested), the probability is very high that some user equipments will not even be able to make a phone call for several tens of minutes.
On the other hand, it is important that all those finding themselves in emergency situations can establish at least one communication within a reasonable time, especially when the impossibility to communicate may cause stress, panic and anxiety, whether on individual or collective levels.
It must therefore be concluded that drawing for the possibility of immediately accessing the network is not appropriate in both the initial transient, i.e., when the access restrictions are applied, and in the steady state, because it makes for unfair access to the mobile telecommunications network.
In addition to the randomness of the draw, some users may also be disfavoured by propagation conditions because, when sending access requests on common channels with possible collisions between concurrent instances, the signals in better propagation conditions will most likely prevail.
Besides the techniques described in the 3GPP specifications, other solutions for limiting the risk of congestion of telecommunications networks are also known, such as those disclosed in patent US 2010/197294 and in patent application WO 2012/013355. The former claims a method for filtering access requests by the network, whereas application WO 2012/013355 describes a method for assigning a network access probability to single user equipments, considering that the network may comprise user equipments having a wide variety of characteristics and different connectivity requirements. Both of these methods assume the existence of a connection between the user equipment and the network, and hence they do not allow for balancing the network access probability when the user equipments can transmit no signals due to access restrictions which are already active.